The invention relates to a digital telephone exchange comprising a plurality of domains each incorporating subscriber line interface circuits, each serving to connect a subscriber line over which digitized speech signals and data signals are transmitted from and to a digital subscriber set, which data signals are both circuit-switched data signals and packet-switched data signals, these last-mentioned data signals comprising signalling information, telemetry data and slow data, and, to connect a time-division multiplex line over which several signals are transmitted from and to the subscriber lines in time-division multiplex, further comprising a time switching network having terminals for several groups of subscriber lines, terminals for auxiliary units for the signalling information and terminals for a domain interface unit for connection to other domains.
For digital telephone exchanges of the abovedescribed type there is the need to convey, in addition to speech signals, circuit-switched data and packet-switched data from and to the subscriber sets. In practice the following provisions are, for example, taken:
1. a 64 kb/s. b-channel for speech,
2. a 0/8/64 kb/s, b'-channel for circuit-switched data, and
3. a 8/16 kb/s delta channel (.DELTA.-channel) for packet-switched data.
The signalling from and to the subscriber sets is usually incorporated in the .DELTA.-channel.
In practice there is also a need for the transmission of telemetry data such as alarm systems for the elderly, meter reading and remote control and slow data for, for example, Teletext and Videotex. It has already been proposed to incorporate these facilities in the .DELTA. channel. The problem is then encountered that telemetry data and slow data have either a high call rate and a short hold period or a low call rate with a very long hold period. Both types of traffic may give rise to congestion in the telephone exchange.